Originally Posted by kOncept
I only use 98 RON "Ultimate" in mine...
$6.00-9.50 difference per fill up or $312-494 per year (assuming you fill up once a week), it's not a big difference considering you're driving around in a $70-80k car?
There are claims that you get improved economy and performance using 98 RON so that already makes up for some of the difference in cost.
I was also in your same shoes when I first upgraded to an e90. But in the long run, you want to avoid running into engine issues - $$$ repair bills later vs incremental fuel cost, you can do the math.
Not sure that the sluggish off the start issue is mainly caused by the type of fuel too. I had the same thing with my n52 (323 though)... but a PBX changed all of that and makes the car more responsive off the start.
Edit: To clarify, I didn't fill up my car with 91 Ron as you OP. Just had the similar thinking off having to pay more for fuel.